Wood panels, generically termed "reconstituted wood panels", include such fabricated products as plywood, flakeboard, hardboard, particleboard, waferboard, oriented strand board and the like. Such panels are manufactured in the form of large relatively thin sheets. Typically, such a panel might be about four feet in width and eight feet in length.
As a consequence of the process of manufacture, such panels display a greater propensity to warpage and sagging than a natural wood product.
It is desirable to be able to establish, at a relatively high speed and in a non-destructive fashion, values for the panel which can be used to calculate a measure of the stiffness of the panel. We have found that the stiffness of a panel correlates with the modulus of elasticity (MOE), and, as a result, with the modulus of rupture (MOR) of the panel.
At this time, to our knowledge, there is no machine available which can, in a matter of seconds, grade a panel by yielding such values. There is, in common use in the industry, a machine known as the Post Flexure machine. In the use of this machine, the panel is threaded on edge into two spaced vertical frames, which frames are then rotated in opposite directions to flex the central part of the panel. The values for the force used to effect rotation, and the extent of deflection effected are used in suitable equations to calculate a value representative of MOE. However, this machine is not suitable for grading on a production line, as it takes several minutes to test a panel with it.
We evolved the following criteria for a machine that could be used to grade wood panels:
(1) it must accommodate the large panels;
(2) it must distribute the deflecting load substantially uniformly and linearly across the full width of the panel, in spite of the warped or sagging nature of said panel;
(3) it must cope with the fact that the load deflection (or moment vs. deflection) curve for a wood panel is not, typically, linear in the proximity of the zero point; and
(4) it must be relatively fast in operation and capable of being upgraded to production line speeds.